MHAI SCOTT, a woman with a history of psychiatric problems, was agitated and behaving erratically at her workplace, a Costco in Sterling, when Loudoun County sheriff’s deputies confronted her two months ago. Ms. Scott was tiny, but she brandished a long-bladed knife and a pair of kitchen scissors in a shopping aisle. Five officers, armed with Tasers and pistols, took up positions in front and in back of her. She rushed two of them, prompting one to run and the other to open fire, striking her with four bullets.

Did Ms. Scott have to die?

The Loudoun County Commonwealth’s Attorney, James E. Plowman, has concluded that the death of Ms. Scottwas a justified homicide. In a lengthy report, he found that the shooting was a reasonable use of deadly force by the officer to defend himself and others.

The report by Mr. Plowman, the county’s top prosecutor, presents the results of a serious and substantive investigation into the May 29 shooting. It includes statements from shoppers and store employees who were witnesses, and from the officers, as well as images from store security cameras and photos of the knife and scissors.

All that is useful, as is the speed with which the report was produced. Still, some critical questions remain unanswered, including the central one: Why couldn’t the officers — five of them, including three who had drawn their Tasers — subdue Ms. Scott short of using lethal force? Would better training have yielded a different outcome?

According to Mr. Plowman’s report, just one of three officers who drew his Taser fired it. The device hit Ms. Scott but apparently malfunctioned. Rather than delivering an incapacitating five-second jolt, which the report says is the norm, the electrical burst lasted just one second, and Ms. Scott was unaffected.

Why? The report is vague, saying it is unclear whether human error or equipment malfunction is to blame. (The Taser in question has been sent to the manufacturer for “more thorough examination,” the report says.) And why didn’t the other two officers who had drawn their Tasers use them? The report says they had reholstered them in anticipation of using handcuffs. But why would they do that when Ms. Scott remained armed?

The confrontation lasted only moments. Ms. Scott refused repeated commands to drop her weapons. The officers had to make split-second decisions. It could not have been easy. Fifteen months earlier, Ms. Scott had had a run-in with police in Ohio, when she yelled incoherently about Satan. After moving to Northern Virginia, she told doctors in March that she had stopped taking her medicines.

But police must be prepared to deal with psychotic, disturbed and aggressive people; that’s part of the job description. Did the deputies in Loudoun receive adequate training for such an encounter? On that question, the report is silent.